A trunk circuit is used in a telephone exchange to superimpose an A.C. voltage on a D.C. voltage. The A.C. voltage is used for signalling and the D.C. voltage is used for controlling the subscriber circuit. Therefore, the potential on the subscriber line must always stay at a positive or a negative polarity potential or alternate between the two, depending on the A.C. and D.C. voltages that are used.
A semiconductor device, such as a thyristor, may be used as a crosspoint or a switching element in the speech path. A characteristic of such a device is that it requires a definite polarity potential relationship and maintenance current, with respect to the subscriber line. More particularly, a thyristor, which requires a self-sustaining gate current for the maintenance of an "ON" state, is easily susceptible to the potential variations which unavoidably occur on the subscriber line circuits because of the capacitive and inductive loads connected thereto, such as the bell coils, which adversely affect the speech path connection.
A proposal has been made to solve such problems by Y. Suzuki et al in an article entitled "A LINE CONCENTRATOR USING ELECTRONIC CROSSPOINTS" published in "The Proceeding of International Switching Symposium '76," Kyoto (October 1976), pp. 243-1-1 to 243-1-5. The proposed approach is based on the selective insertion (for the A.C. signal transmission period) of a resistor across either the subscriber line pair or between an outgoing signal terminal of the pair and either ground or a negative voltage source. The inserted resistor constitutes a stable path for supplying the sustaining gate current of the thyristors, during the period in which the A.C. signal passes through the thyristors. However, such a circuit requires a high D.C. voltage to enable the gate electrodes to drive the thyristors, thus making the telephone exchange system more costly to manufacture, than it otherwise has to be.
Another approach is disclosed in U.S. Pat. No. 3,959,668 which issued to I. Ohhinata et al and is based on a combination of a pair of cathode-gate drive thyristors driven by supplying current and a pair of anode-gate drive thyristors driven by taking out current. These thyristors are arranged in such a manner that at least one of the gate currents flows as long as the A.C. signal passes through the thyristors. However, this requires four drive means for each subscriber line pair, thus increasing the complexity of the speech path switching elements.